Units and Dimensions
In the Properties Window > Scene > Units you can define what units your scene is in, and how dimensions are processed, although it is pretty irrelevant and won't really make any difference.

Then in the 3D view you can press the N button to open the Properties Region, there you have the Transform Panel where you can see the real dimensions of an object.

If you edit it don't forget to apply the scale to the object with Ctrl + A, so you are always working with evenly scaled objects and don't get unexpected results in Edit Mode.

Lastly in Edit Mode, under the Properties Region there will be a panel available called Mesh Display.
There you can turn on Edge Length option, so you can see the dimensions of your mesh.

You can also use Snap to Increment with the Absolute Grid Alignement option turned on to stick to an orthogonal worldspace grid.